1. Field of the Invention
This invention pertains to a velocity change sensor or accelerometer used in motor vehicles for detecting sudden changes in velocity and for activating in response a passenger restraining device such as an air bag. More particularly, this device includes an element which moves to a preset position in response to a sudden deceleration to activate a pair of contact blades.
2. Background of the Invention
Studies have been made which indicate that injuries in motor vehicle accidents, especially at high speeds, can be substantially reduced or eliminated by the use of passenger restraint systems. (The term "passenger" is used to cover the driver of a car as well.) These systems include an inflatable balloon usually termed an air bag which normally is stored away in the instrument panel or the steering wheel. When the motor vehicle is subjected to a sudden deceleration, the air bag is inflated and is deployed automatically in a position which cushions the passengers, restrains their movement and prevents contact between them and the automobile interior such as the windshield, the steering wheel, the instrument panel and so on. Of course, a crucial element of all such systems is the velocity change sensor or accelerometer which initiates the inflation and deployment of the air bags. The motion of the motor vehicle must be carefully and precisely monitored so that the air bags can be deployed very fast, before the passengers suffer any substantial injury.
A velocity change sensor is disclosed in U.S. Pat. No. 4,329,549 assigned to the same company as the present invention. This sensor comprises a tubular housing surrounding a metallic shell, a metal ball and a magnet biasing the ball toward a first end of the shell. At the second end of the shell there is a pair of electrical contact blades. The sensor is positioned in the motor vehicle in an orientation such that when the motor vehicle experiences a deceleration which exceeds a preset level, the ball moves from the first toward the second end, making contact with the two blades. Because the blades and the ball are made of on electrically conducting material, when the ball contacts the blades, an electrical path is established between the two blades. This electrical path is used to initiate a signal for the deployment of the air bags.
One problem with the device shown in U.S. Pat. No. 4,329,549 is that the electrical contact is made through the ball, and since the contact points are indeterminate the whole surface of the ball must be treated to use a low resistance electrical path. For example, the ball is often coated with a thin layer of gold or similarly, highly conductive material. However, this process is expense and time consuming. Furthermore, the conductive layer may wear off as the ball moves along its path.